1. Field of the Invention
This invention is in the field of non-destructive testing of magnetizable workpieces for nonhomogeneities in which an indicating composition in the form of a dispersion of ferromagnetic particles and fluorescent pigment particles is preferably sprayed onto the magnetizable workpiece which has been preheated in a hot water bath.
2. Description of the Prior Art
Steel billets which represent the last semifinished intermediate step between the ingot and the finished shape have been subjected to numerous types of non-destructive testing on a continuous basis, usually at a substantial velocity. A typical example of such testing procedures using magnetizable particles will be found described in U.S. Pat. No. 4,130,800.
Magnetic particle inspection in which magnetizable particles in the form of a suspension in a fluid or in dry form are applied to the surface of a magnetized piece have been in commercial use for many decades. In the inspection of billets, it is particularly important to locate seams which are longitudinal discontinuities that appear as light lines in the surface of the steel. Seams normally are closed tight enough so that no actual opening can be visually detected without magnetic particle inspection. Seams can result from a large number of sources, some of which are mechanical and some metallurgical. Brush seams are clusters of short seams that appear as though they had been painted or brushed onto the surface. Usually, these defects are the result of removal of metal from the steel surface by scarfing or scaling, exposing ingot blow-holes and the subsurface porosity. The brush type seams may range in depth from about 0.005 to 0.300 inch and may occur either in zones or across the entire surface of the billet.
Seams can also be caused by strings of non-metallic inclusions, or metallic inclusions which have a different permeability from the parent metal. Seams can also be caused by overfill or underfill in the rolls, or by cold shuts due to an overlapping of the metal from splashing in the mold.
Seams vary in difficulty to detect by means of magnetic particle inspection. Some seams are able to cause a very abrupt change in permeability in which case they are relatively easily located by conventional magnetic particle inspection. Other seams, particularly those which occur in alloy steels containing chromium and molybdenum, for example, are apparently much more difficult to locate.
In more recent time, use has been made of magnetic particles to which fluorescent pigments are attached. The inspection of the pieces is then facilitated by examining the same under ultraviolet or black light to locate the presence of occlusions of the magnetic particles. Prior art patents referring to this type of inspection technique include U.S. Pat. Nos. 2,267,999 and 2,936,287. These patents relate, respectively, to lacquer bonded and resin bonded fluorescent magnetic particles for use in this type of inspection process.
Steel billets are normally inspected in a continuous process in which the billet travels at relatively high velocity through a magnetizing yoke and through an applicator station where the fluorescent type magnetic particles are applied either in suspension or in dry form. Throughout the inspection process, the billet is likely to be handled roughly with the result that the occlusion of magnetic particles at seams and other flaws may be disturbed or even destroyed by such rough handling.
In conventional installations, the system includes a recirculating aqueous fluorescent magnetic particle bath. The inspection is carried out under black light in the dark by inspectors who mark the indications with large white or yellow crayons. The billet thus marked is then sent to scarfers or grinders who locate the seams and burn or grind them out at a conditioning station.
This type of technique has several disadvantages. Because the fluorescent magnetic particle suspension is constantly recirculated, it picks up a substantial amount of mill scale and returns it to the magnetic particle bath. Unless they are specifically descaled, billets always have a heavy, loose brittle coating of mill scale which is formed as the red hot billet cools in air. In units using the recirculating bath, substantial quantities of the scale get into the bath and are ground up. Since the mill scale is magnetic, it competes with the magnetic particles for flaw sites, and in addition masks the fluorescence and dims it appreciably. Studies have shown that typical baths may contain five to ten times as much ground up mill scale as the fluorescent magnetic particles.
Another problem which has been presented in the past is caused by seasonal change in temperatures. In several months of the year, the temperature of the billet is below the freezing point of water so that the recirculating baths have to be protected against freezing by adding an anti-freeze material such as ethylene glycol or by heating the tanks and all the plumbing. However, glycol solutions at temperatures below freezing are extremely viscous, thereby retarding the formation of indications and also increasing the dragout of the expensive glycol solution.